1. Technical Field of the Invention
The present invention relates to a structure of a magnetic bearing that supports a rotor without making contact, particularly a stator core for a homo-polar type of magnetic bearing, and a method of manufacturing it.
2. Prior Art
A turbo compressor can be made larger in capacity and smaller in size than a reciprocating or screw compressor, and can be easily made to an oil-free type. Therefore, turbo compressors are used often as general-purpose compressors in applications such as a compressed air source for factories, a source of air for separation, and other various processes.
Conventionally, gas bearings, sliding bearings and magnetic bearings have been used to support a high-speed rotating shaft of a high-speed motor that is connected directly to and drives a turbo compressor. In particular, a homo-polar magnetic bearing can be used to support a rotor (rotating shaft) in a contact free manner that rotates to form the high speed shaft of a high speed motor by passing magnetic flux through the shaft to produce an electromagnetic sucking force which causes the shaft to float, this being one type of radial magnetic bearing for use with shafts that rotate at a high speed (for instance, 100,000 min−1 or more).
FIGS. 1A and 1B show typical schematic viewes of a conventional homo-polar magnetic bearing. In these figures, a homo-polar magnetic bearing 1 is composed of a rotor 3 that is arranged at the axial center of a casing 2 and parallel to it in the axial direction and can rotate at a high speed, U-shaped stator cores 4 installed inside the casing 2 with gaps between the outer surface of the rotor 3, and coils 5 that are placed around the toothed ends of the stator cores 4.
In addition, a plurality of stator cores (4 cores in FIGS. 1A and 1B) are disposed equally spaced in the circumferential direction with gaps between the outer surface of the rotor 3. Although not illustrated, stator cores 4 are arranged in the axial direction of the rotor 3 in at least 2 locations with a predetermined distance between them. Consequently, the rotor can rotate stably at a high speed. A stator core 4 is made of laminated steel sheets each of which is manufactured with an insulating adhesive material applied to its surface to bond to an adjacent thin steel sheet, and these are bonded one after another to obtain a predetermined length. As shown in FIGS. 1A and 1B, the direction A in which the laminated steel sheets 4 (lamination) are bonded is arranged to be perpendicular to the axial direction Z of the rotor 3.
As described above, in the homo-polar magnetic bearing 1, since the toothed ends of the stator cores 4 that surround the rotor 3 are close to each other in the axial direction and as the coils 5 produce the N and S poles of an electro magnet, the homo-polar magnetic bearing 1 can float the shaft in a contact free manner and support the rotor 3 by the sucking force of the toothed ends located opposite each other. Therefore, the direction of this homo-polar magnetic field is parallel to the centerline of the rotor and on the outer surface of the rotor 3 as shown by the dashed arrow lines in FIG. 1B.
FIG. 1C is a schematic view that shows a conventional process for assembling laminated steel sheets to form a conventional stator core. Normally, the stator core 4 of the homo-polar magnetic bearing 1 is manufactured by making thin rectangular steel sheets 4a coated with an insulating material, by a method such as punching, and assembling these punched steel sheets 4a one after another, to produce a laminated stator core 4.
However, when the inner surfaces of the aforementioned stator cores 4 (laminated steel sheets) are cut by a rotary cutting process, a large cutting load is applied to the edges of the laminated steel sheets 4a in a lateral direction, so the tips of the laminated steel sheets 4a are bent, and the insulating material is crushed in the direction of rotation by the above-mentioned bending load, which is a practical problem. Consequently, the steel sheets contact each other resulting in an increase in the eddy currents in the stator unit, so another problem occurs due to the reduced levitation force applied to the rotor 3, poor rotating characteristics, etc. Still another problem is that the laminated material is peeled away by the edge of the cutting tool. Even if the above-mentioned process of cutting in a lathe is replaced by using a vertical boring machine etc. to cut the inner surfaces of laminated steel sheets, because there are gaps between adjacent steel sheets, there is the additional problems that smooth cutting and true roundness cannot be easily ensured.
On the other hand, the inventors of the present invention have proposed the homo-polar magnetic bearing apparatus configured as shown in FIGS. 2 and 3, with the aim of improving the characteristics of conventional homo-polar magnetic bearings (unpublished Japanese patent application No. 88402/2000). According to this magnetic bearing apparatus, adjacent N poles or S poles are connected together in the circumferential direction, or are located close to each other with a small gap between them. The homo-polar magnetic bearing with this configuration has the advantage that it is capable of greatly reducing the production of eddy currents and the heat and eddy current losses generated in the rotor.
However, if the stator cores 4 of the homo-polar magnetic bearing shown in FIGS. 2 and 3 are produced using laminated steel sheets with small eddy current losses, as shown in FIG. 1, the laminated steel sheets become so thin in the peripheral web 4b that they fail, crush or peel when processed, which is a practical disadvantage.
More explicitly, in the homo-polar magnetic bearing with the structure shown in FIGS. 2 and 3, the stator cores 4 are connected together circumferentially or located close to each other, so the distribution of magnetic flux in the rotor is more uniform and losses can be reduced. Conversely, however, if stator cores 4 in which the tips are connected together are formed with a conventional laminated structure, the laminated steel sheets are so small in the portions where adjacent magnetic poles are connected together that the laminated structure may collapse when the cores are machined, therefore, it is very difficult to machine the cores without detaching, crushing or peeling the laminations.
Another problem in a conventional apparatus is that amorphous materials cannot be used because they are difficult to laminate, despite the advantages of having a high electrical resistance and permeability, so the choice of electromagnetic sheet steel is restricted.
Next, the structure of a conventional homo-polar radial magnetic bearing is described in more detail than before by referring to FIGS. 4 and 5. FIG. 4a is a front view of a conventional homo-polar radial magnetic bearing, and FIG. 4b is the corresponding side sectional elevation. FIG. 5 is an isometric view of the stator core of a conventional homo-polar radial magnetic bearing.
The homo-polar radial magnetic bearing 1 is provided with a casing 2, a plurality of electromagnetic components 13 and a rotating shaft 3. The rotating shaft 3 is made of a material which is magnetic at least on the surface thereof, with an outer diameter of D1 and a length determined by the rotor. The rotor 3 is disposed coaxially with the centerline of the casing 2, parallel thereto in the longitudinal direction, and is supported so that it can rotate freely. The plurality of electromagnetic components 13 support the rotor 3 so that it can rotate freely, and are arranged around the rotor 3. For instance, four electromagnetic components are connected together to form a set, and sets of electromagnetic components 13 support the rotor 3 at 2 locations. At each supporting location, 4 electromagnetic components are equally spaced around the rotor.
The electromagnetic components 13 are provided with stator cores 80 and coils 5. The stator core 80 is provided with two yokes 6 and 8 and a stem portion 7 as shown in FIG. 5. A yoke 6 or 8 is a column-shaped portion one end of which is opposite the outer surface of the rotor 3 with a gap between them that induces a magnetic pole on the surface 9. The two yokes 6, 8 are arranged axially with a predetermined spacing between each other. The stem portion 7 is a magnetic structure between the other ends of the two yokes 6, 8 connecting the yokes together. The stator core 80 is a thick U-shaped unit comprised of the two yokes 6, 8 and the stem portion 7 without gaps, and is installed in a recess on the inner periphery of the casing 2.
The coil 5 is a bundle of wire. The wire is wound in several layers around the yokes 6, 8 with an air gap between the coil and yoke. The coil 5 is a block with the same shape as the section of the yoke 6 or 8 with an air gap between the coil and yoke.
The structure of the stator core 80 is described in further detail referring to FIG. 5. The stator core 80 is made of laminated steel sheets, consisting of a plurality of magnetic steel sheets 81 and an insulating material. The magnetic steel sheet 81 is a thin steel sheet with a thickness T, shaped in the aforementioned U shape. The insulating material is a non-conducting material and is applied between the plurality of magnetic steel sheets 81. When the stator core 80 is assembled as an electromagnetic component, it is laminated in the circumferential direction of the rotor. The magnetic steel sheet 81 of the illustrated stator core 80 is rectangular in shape with a width W1 and a height H1, provided with a slot W2 wide and H2 in height, on the side forming the magnetic pole surface 9. The stator core 80 is made of a plurality of laminated magnetic steel sheets 81 with a predetermined length of L1.
In another type of electromagnetic component, the width of a stator core 80 near the magnetic pole surface 9 is extended circumferentially in the direction of the outer surface of the rotor, and comes in close contact with the magnetic pole surfaces of the adjacent electromagnetic components of the stator core.
According to still another type of electromagnetic component, the width of a stator core 80 near the magnetic pole surface 9 is extended circumferentially in the direction of the outer surface of the rotor, and is integrated with the magnetic pole surface of an adjacent electromagnetic component of the stator core 80.
When the aforementioned stator core for a magnetic bearing is manufactured, thin sheet steel with a thickness T is punched using dies, to produce U-shaped magnetic steel sheets.
Next, the magnetic pole surface 9 of the stator core for a magnetic bearing must be machined into a circular arc using a lathe etc.; at this time, the rotation causes a cutting load that acts laterally on the edges of the laminated steel sheets, so the tips of the electromagnetic steel sheets are bent; due to this bending, the insulation material is crushed in the direction of rotation, often resulting in adjacent electromagnetic steel sheets coming in contact with each other. The problem encountered when this happens is that large eddy currents are produced in the electromagnetic steel sheets.
Another problem that the laminated steel sheets become separated during cutting, may occur.
There is also another problem that if a vertical boring machine is used instead of a lathe, differences are produced at the edges between adjacent laminated steel sheets, and a true, smooth circle cannot be ensured.
With the type of stator core for a magnetic bearing in which the magnetic pole surface of the stator core is extended over the outer surface of the rotor, since the laminated steel sheets in the extended portions become very thin, they may cause problems by becoming detached, crushed or peeled during machining.